1. Field of the Invention
The present invention relates to a method of depositing pyrolyzed films having improved performance and a glazing pane coated with the same.
2. Description of the Background
It is known, from EP 125 153, for example, that a low-emissivity film can be formed on a glass substrate from a tin compound in powder form, especially from dibutyl tin fluoride (DBTF). The deposition is carried out at a high temperature in the region of 550.degree. C. This deposition may be performed at a speed sufficiently high for coating a ribbon of float glass, of whatever thickness, that is to say at a speed which may range from 3 to more than 25 m/min and which on average, for glass thicknesses most commonly produced at present, is about 12 m/min.
When a film of SnO.sub.2, doped with fluorine, of a thickness of about 180 nm is produced as described in EP 125 153, an emissivity of the order of 0.35 is achieved. With an increased thickness of from 300 to 350 nm, an emissivity of the order of 0.25 is attained.
This 180 nm film is slightly bluish in reflection, and can tolerate only small variations in thickness, of the order of the size of the DBTF molecule, if variations in color are to be avoided. Thus, if the thickness increases too much, the color becomes yellow.
The film of 300 to 350 nm thickness is slightly green in reflection and if its thickness varies too much, its color also varies. Thus, if its thickness increases, the color of the film may become red.
Therefore, film thickness must be carefully controlled so that the color of the film will not vary locally if the thickness is not uniform, or overall if the thickness remains uniform. Moreover, the selection of desirable performance characteristics for a product cannot be made without considering that performance and thickness, and therefore color, are linked together. It is possible to remove these constraints, imposed by color, by selecting film thickness ranges for which the colored appearance in reflection disappears. Certainly, interesting emissivity levels are achieved, of the order of 0.12 and even less, from 800 or 1,000 nm. However, the light transmission through the glazing is reduced, production costs are increased by the much greater consumption of coating material, and the dull, veiled appearance of the film, approximately proportional to its thickness, increases. Further, production difficulties increase (for example, fouling of the powder projection nozzle increases due to the higher powder flow rates), and, hence, cleaning and stoppages become more frequent. Further, the glass is much more cooled, and, hence, pyrolysis is less effective. Additionally, process limits are also approached, and greater amounts of pollutants are produced.
It is also known that the pyrolysis efficiency can be increased if the deposition is carried out on a hotter glass, but this leads to defects of planeity in the glass by reason of its increasing deformability with temperature. In fact, if the glass were raised to too high a temperature, it would soften and sag between the supporting rollers conveying it.
Thus, a need exists for a method of depositing pyrolyzed films having improved performance and a glazing pane coated with the same.